Fix wxSearchCtrl::HasFocus() in the generic version.

Override GetMainWindowOfCompositeControl() in the windows composing the
generic wxSearchCtrl to ensure that HasFocus() correctly returns true for the
entire control when any of them has focus.

Add a trivial unit test to check that this really works and facilitate adding
more tests for this class later.

Closes #14970.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73401 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/test_test_gui.dsp

@@ -477,6 +477,10 @@ SOURCE=.\..\samples\sample.rc
 # End Source File
 # Begin Source File
 
+SOURCE=.\controls\searchctrltest.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=.\misc\selstoretest.cpp
 # End Source File
 # Begin Source File